MantisBT has a Content Security Policy bypass via attachments

Given any pre-existing XSS / HTML injection vulnerability, an attacker can bypass the Content Security Policy's script-src directive by uploading a crafted attachment to any issue that, when accessed via the file_download.php link, will be downloaded with a valid JavaScript MIME type resulting in script execution.

The uploaded payload must be sniffed as a valid JavaScript MIME type by PHP finfo (see file_create_finfo() API function). Non-JavaScript MIME types will not get imported in a <script> tag by the browser, due to response header X-Content-Type-Options being set to nosniff, which requires all imported JavaScript files to be a valid JavaScript MIME type.

Impact

Cross-site scripting

Patches

• 9e3bee2e7b909f4e3596985892b8bc8bee9e0bfe

Workarounds

None

Credits

Thanks to siunam (Tang Cheuk Hei) for discovering and responsibly reporting the issue.